Tropical forests are approaching critical temperature thresholds.

Area of Science:

• Plant physiology and climate change impacts on tropical ecosystems.

Background:

• The critical temperature (Tcrit) for tropical tree photosynthesis is around 46.7°C.
• Current leaf temperatures and projected climate change impacts on reaching this threshold remain uncertain.

Purpose of the Study:

• To assess current and future leaf temperatures in tropical forests relative to photosynthetic critical temperatures.
• To determine the potential air temperature increase tropical forests can tolerate before metabolic function tipping points.

Main Methods:

• Utilized leaf thermocouples, pyrgeometers, and remote sensing (ECOSTRESS) for pantropical canopy temperature measurements.
• Conducted controlled leaf warming experiments (+2-4°C) in Brazil, Puerto Rico, and Australia.
• Developed and validated an empirical model using experimental data to predict metabolic tipping points.

Main Results:

• Midday peak canopy temperatures reach ~34°C during dry periods, with tails exceeding 40°C.
• Upper canopy leaves exceed Tcrit 0.01% of the time under current conditions.
• Warming experiments showed peak leaf temperatures exceeding Tcrit 1.3% of the time, with higher percentages at elevated temperatures.
• Empirical model predicts a 3.9 ± 0.5°C air temperature increase tolerance before a metabolic tipping point.

Conclusions:

• Tropical forests face a critical tipping point with a potential 3.9°C air temperature rise.
• This estimate aligns with worst-case climate change scenarios (RCP 8.5).
• Mitigating climate change (e.g., avoiding RCP 6.0 or 8.5) is crucial for the survival of these vital ecosystems.